1. Field of the Invention
This invention relates to an optical waveguide device, and more particularly to an optical waveguide device with diffraction gratings, which are arranged in such a manner that a guided wave, after being diffracted by the gratings, exits from the optical waveguide.
Moreover, the present invention relates to an optical waveguide device with diffraction gratings, which are arranged in such a manner that an external wave of light, after being diffracted by the gratings, enters the optical waveguide device.
2. Description of the Prior Art
Heretofore, there have already been proposed several types of optical waveguide devices in which a wave travels in a guided mode along an optical waveguide laid on a substrate. In such a type of optical waveguide device, grating couplers, formed on the surface of the optical waveguide, are commonly employed in causing a guided wave to be emitted from the optical waveguide, or an external wave to be introduced into the optical waveguide.
The grating couplers are advantageous in that an incoming or an outgoing wave tends to suffer less deterioration in optical quality and to be stable when it enters, or is emitted from, the optical waveguide. However, it is also well admitted that this type of grating couplers have drawbacks such as a low entrance/emission efficiency of the wave, and that the variation of the efficiency is sensitively dependent on errors in the dimensions of the grating couplers, that is, errors in the ratio of pitch to line width.
In order to improve the entrance/emission efficiency of the wave, attempts have been made to effectively reduce the number of coupled beams in the grating coupler. Examples of such attempts include the arrangment shown in the Journal of the Optical Society of America Vol. 63, No. 11, pp. 1419-1431. In this arrangement, grating couplers are formed on the surface of an optical waveguide, which surface is located on the substrate side, on a transparent substrate. With these grating couplers, a guided wave is diffracted into the air side and the substrate side, and the wave of a minus-first order diffracted toward the substrate is reflected from the reflecting layer on the surfaces of the substrate. Thus, the reflected wave interferes with the minus-first order wave diffracted to the air side, thereby intensifying each other. This arrangement, which principally effects the coupling of two beams, prevents the distribution of an optical power into two directions: the substrate side and the air side, because the waves effectually exit only to the air side, thereby realizing high coupling efficiencies.
However, this arrangement requires extremely strict conditions in coupling the two beams in order to obtain considerably high coupling efficiencies, which in turn require grating couplers of submicrons or less in width, and which render the aspect ratio of the grating coupler difficult to control. Therefore, the grating coupler of this type is very difficult to manufacture.
The above-mentioned literature also relates to the fact that a minus-second order wave, as well as a minus-first order wave, are diffracted into both the substrate and the air. Particularly, this literature discloses the arrangement in which the minus-first order wave diffracted to the substrate is reflected from the reflecting layer on the surface of the substrate to interfere with the minus-first order diffracted wave directed to the air in such a manner as to intensity each other, and in which the minus-second order wave diffracted to the substrate is reflected from the reflecting layer on the surface of the substrate to interfere with the minus-second order wave oriented to the air so as to weaken each other. This arrangement will be inferior in efficiency due to the coupling of four beams without the reflecting layer, but can lead to high coupling efficiences when the reflecting layer is employed because the distribution of the optical power to the minus-first order diffracted wave is effectually improved by means of the reflecting layer.
This arrangement results in the pitches of the grating couplers being less smaller, but the resultant pitches are still not more than 1 .mu.m, which renders the fabrication of the grating couplers difficult. Moreover, even with this arrangement, the precise regulation of the aspect ratio is difficult to practice, thereby rendering the control of the coupling length between the grating couplers and the optical waveguide difficult: that is, entailing differences in the diameter of an outgoing beam and input efficiencies.
Another example is disclosed in U.S. Pat. No. 4,691,982. In this example, grating couplers are formed on the surface of an optical waveguide laid on a transparent substrate: that is, the surface of the optical waveguide being much closer to the air side. In addition, a cladding layer (a buffer layer) is further provided over the grating couplers. The cladding layer is then covered with a relecting lyaer.
With this structure, a wave diffracted by the grating coupler toward the substrate interferes with a wave which is first diffracted toward the cladding layer and is subsequently reflected from a boundary, between the cladding layer and the reflecting layer, back to the substrate, thereby intensifying each other, and then the resultant wave emerges from the substrate.
This arrangement, however, probably fails to achieve the high efficiency because it lacks a definite number of beams, to be coupled with a guided wave, and the specific order of diffracted waves being used.